One of processes of manufacturing a vehicular lamp is a process of integrally fixing a transparent front cover to a container-shaped lamp body. As for a technology for such fixation, a technology of welding the lamp body and the front cover is used. Specifically, the transparent front cover is positioned to be abutted on the lamp body and a laser beam is projected to abutted surfaces from the front cover side so that the front cover and the lamp body are molten and joined to each other at the abutted surfaces by the light energy of the laser beam. Another welding technology is also known in which in order to enhance the welding quality by the above-described welding method, the welding is performed in a state where the abutted surfaces are more closely contacted with each other by installing a plate-shaped transparent jig at the front cover side and pressing the front cover against the lamp body by the transparent jig.
However, when the laser welding is performed using the transparent jig, a spot diameter may be increased when the laser beam projected to the welding portion has passed through the transparent jig and the front cover. This may cause degradation of the welding quality. That is, the projected laser beam is refracted at a surface of the front cover or the transparent jig. Due to the refraction, the spot diameter of the laser beam is increased on welding surfaces. When the spot diameter increases, a light energy per unit area (hereinafter, referred to as a “light energy density”) decreases, thereby degrading the welding quality. Japanese Patent Laid-Open Publication No. 2006-12502 proposes a technology that equalizes the sum of the thicknesses of the transparent jig and the front cover in order to prevent a variation of the spot diameter of the laser beam due to a variation of the sum of the thicknesses of the transparent jig and the front cover.